CMPL SEMICON
National Institute of Physics
University of the Philippines - Diliman
Active research is being done on GaAs and Si solar cell devices to improve its cost and efficiency. The Condensed Matter Physics Laboratory-Semicon group is also in search for ways to fabricate high quality GaAs and Si solar cells. The research involves from the synthesis of materials, device fabrication and testing of the devices.
Basically, a solar cell is a device that converts energy from the light of the sun to electrical energy used by humans. Solar cells are considered to be ecofriendly since there is no harmful waste product from the energy conversion. Thus, it continuously supplies the world with green energy. Helping us with our everyday need for electricity without harming the environment.
Metallization
GaAs and Si are semiconductors that react to the spectra of light emitted by the sun. These materials generate electron-hole pairs that when harvested produce electrical power. There are different designs for these devices. But in its simplest form, the cell is just composed of a PN-junction and electrical contacts. These cells are hooked onto batteries and inverters so that the electrical energy harvested can be utilized for everyday use.
Metallization in solar cells involves the use of high vacuum chambers. The high vacuum environment frees the deposition of impurities. Itis necessary to deposit high quality metal films since the performanceof this device is related to the purity of the metal contact. Vacuum is achieved with the use of pumps such as rotary and turbomolecular pumps. For gallium arsenide (GaAs) based devices, the ideal contact wouldbe gold since it is resistant to oxidation and has very low electrical resistance. Aluminum and silver is usually used for silicon solar cells. To produce ohmic contacts for GaAs devices, metal alloys such as gold-germanium (AuGe) and gold-beryllium (AuBe) are utilized for dopedcrystals.
Deposition of gold is done using resistive evaporation. By passing a high current across a metal pellet. The intense heat causes the solidmetal to vaporize. The metal is then deposited as a thin film on thesubstrate. A quartz crystal monitor monitors the thickness of thedeposited film during deposition. Silver and aluminum can also be deposited through this process. Aluminum can be also be deposited via sputtering.
Sputtering utilizes nonreactive gas ions to deposit thin metal films on differentsubstrates. The deposition process uses nonreactive gas such as argon.The gas is ionized then guided towards the metal target. Continuouscollisions in the metal target surface cause particles to be ejected thendeposited on the substrate. The table on the right shows exampls of metals used as contact on their respective semiconductor.
